In recent years, secondary batteries have been widely used as power suppliers to an electric vehicle, a hybrid electric vehicle, and a battery-assisted bicycle, or power suppliers to electric assemblies. For example, since a rechargeable lithium-ion battery, for example, a nonaqueous secondary battery has a high output and a high energy density characteristics, the rechargeable lithium-ion battery attracts attention as power supplies to the electric vehicle.
Generally, the secondary battery is constituted as a cell equipped with an exterior container formed in a shape of a flat rectangular box made from aluminum etc., a group of electrodes contained with an electrolyte in the exterior container, and electrode terminals formed in the exterior container and connected to the group of electrodes.
A battery unit, in which a plurality of cells are arranged side by side in a case and connected in parallel or in series in order to attain a high capacity and a high power characteristics, is used. Furthermore, a secondary battery assembly in which an electric circuit is equipped in the battery unit is also used. For example, as disclosed in Japanese patent application Laid Open No. H7-237457, an exterior case of such secondary battery assembly is constituted by two or more firm frames which bundle, hold or support the cells, or by a firm exterior case which covers all the cell groups.
However, in the above secondary battery assembly, when the frames and the exterior case are formed of firm metals, such as steel, the secondary battery assembly tends to become heavy. Furthermore, since there are many chances to centralize strength to bind the battery units, a connecting portion is grown in size, and the assembly process becomes complicated. For example, in the electric vehicle etc., when aiming at a damage reduction of roads or an improvement in continuous driving mileage, a miniaturization and a high density package of the secondary battery assembly have been a big issue.